METABOLISM 285 



benzoic, salicylic, &c., but the nature of the processes 

 which give rise to them is not well ascertained. 



Certain decomposition products of cellulose may also 

 be mentioned here. The lignin and suberin which are 

 characteristic of woody and corky cell-walls arise in this 

 way. During their formation, which takes place in the 

 substance of the cell-wall, they can be removed by appropriate 

 solvents, leaving the cellulose skeleton which they have 

 been gradually replacing. These differ from most of the 

 substances described in that they can be produced in the 

 walls of cells that have lost their protoplasm, so that their 

 formation is not directly dependent on metabolism. 



We have again the odorous substances, and the colour- 

 ing matters other than those already mentioned. Many 

 colouring matters are products of the decomposition of 

 chlorophyll, especially certain of those to which the 

 autumnal tints of leaves are due. One of this group, 

 xanthophyll, is a bright yellow pigment which is always 

 associated with the chlorophyll, though in varying amount. 



We have finally in connection with the metabolic pro- 

 cesses to touch upon the excretions of plants. The term 

 must be used in a wide sense to include all such sub- 

 stances as are undoubtedly withdrawn from the seats of 

 active life, whether thrown off from the plant-body or not. 

 The excreta which are completely eliminated are few ; 

 under normal conditions only the carbon dioxide and water 

 which are products of respiration can be specified. Under 

 abnormal conditions volatile compounds of ammonia or 

 ammonia itself may be added to these. But there are 

 certain other substances which are thrown off by a few 

 plants, and for them some of these subserve definite 

 purposes. Perhaps the most frequently occurring instance 

 of these is the sugary solution known as the nectar, which 

 is so common in flowers, and which is excreted usually to 

 serve as an attraction to insect visitors. Mineral matters 

 such as calcium carbonate are in some cases excreted on to 

 the surface of the leaf, sometimes by special glands as in 



